Your Guide to Fixing Under-Extrusion on a 3D Printer

Under-extrusion is when a printer lays down less material than the G-code calls for, producing gaps, thin infill and weak walls. Fix it in this order: flow rate at 100%, correct filament diameter, temperature up in 5 °C steps, then nozzle, extruder gear and feed path. Software causes are more common than hardware ones.

You have waited hours for a print, pulled it off the plate, and found something starved-looking: gaps between lines, wispy infill, walls you can push a fingernail into. It is one of the most common faults in 3D printing, and it is almost always fixable in the order above — cheapest checks first, tools last.

What is Under-Extrusion and How Do You Spot It?

Under-extrusion happens when your 3D printer cannot push out the volume of filament the job requires. The toolhead follows the correct path but does not deposit enough material, which compromises both appearance and, more importantly, structural strength.

Here are the classic signs of an under-extruding 3D printer:

• Gaps between adjacent lines on top surfaces and walls.
• Entire layers missing — see our guide to missing layers if this is your main symptom.
• Thin, wispy or incomplete infill structures.
• A rough, pitted or hole-covered surface texture.
• Prints that feel noticeably brittle and snap along layer lines.

Symptom to Cause to Fix

Under-extrusion has one appearance and half a dozen causes. Match the pattern before you start changing settings.

Start Here: Are Your Slicer Settings Causing Under-Extrusion?

Always check the slicer first. The fixes are fast, free, and solve the problem outright more often than not. An incorrect setting in slicing software is a frequent cause of 3D printing issues.

How to Calibrate Flow Rate Properly

Guessing at flow rate is how people end up chasing under-extrusion for weeks. Measure it instead:

1. Print a hollow 25 mm cube with 0 top layers, 0 bottom layers, 0% infill and exactly 2 perimeters.
2. Measure the wall thickness with digital calipers at six points, avoiding the corners and the seam.
3. Compare the average to the expected value: 2 perimeters at 0.42 mm line width should measure 0.84 mm.
4. Multiply your current flow rate by expected divided by measured. If you measure 0.80 mm at 100% flow, the corrected value is 100 × 0.84 / 0.80 = 105%.
5. Re-print and re-measure once to confirm. Anything outside roughly 92–108% suggests a hardware fault rather than a calibration offset.

Our wider guide to calibrating a 3D printer covers the other tests worth running alongside this one.

Speed, Layer Height and the Flow Ceiling

Above a certain speed, under-extrusion is physics rather than a setting. The volume a hotend must melt per second is line width × layer height × speed, and every hotend has a ceiling:

Very few 0.4 mm hotends of any brand sustain 50 mm³/s, which is why a headline speed figure is never a sustained extrusion figure — slicers cap the actual speed by volumetric flow for exactly this reason. If a model under-extrudes only on fast infill, either raise the temperature to thin the melt or lower the speed for that feature. Moving to a 0.6 mm nozzle raises throughput far more effectively than raising the speed number does. Current QIDI machines all use a 0.4 mm bimetal nozzle rated to 370 °C with 0.6 and 0.8 mm available; the figures for each model are on the Plus 5 tech specs page.

Is Your 3D Printer's Hardware the Problem?

If your slicer settings are confirmed correct and the problem persists, inspect the physical hardware.

Check for a Clogged Nozzle

• What to look for: a partial clog is far more common than a full one. You may see 3D printer filament curling to one side as it exits, or feel extra resistance pushing filament through by hand.
• Solution: perform a cold pull. On a QIDI Q2, Plus 5 or Max4, heat to 250 °C, hand-feed PLA until it extrudes cleanly, cool to 90 °C, then pull firmly. Our nozzle jam guide covers the full escalation, and general clog troubleshooting is worth a read too.

Inspect the Extruder Assembly

• What to look for: the hobbed gear can pack with plastic dust and start slipping. The tension arm may be too loose or too tight.
• Solution: clear the teeth with a small brass brush. Set tension so the gear grips firmly and leaves light marks on the filament without crushing it into an oval. Spares live in Q2 accessories.

Examine the Filament Path

• What to look for: tangles on the spool, or a guide tube that is sharply bent, kinked or worn inside.
• Solution: ensure the spool rotates freely. On a Bowden system, remove tight bends and replace a worn PTFE tube. Direct-drive machines have a much shorter path, which is one reason they suffer less from this failure mode.

Check for a Worn Nozzle

• What to look for: an orifice that has worn open produces the opposite symptom on walls (they measure oversize) but starves top surfaces, because flow is calculated for the diameter the profile assumes.
• Solution: replace it, and match the material to your filament. Composites need hardened or bimetal nozzles; stock spares are in Q2 bimetal nozzles.

Could Your Filament Be the Hidden Culprit?

If software and hardware check out, the filament itself may be the root cause. Low-quality or poorly stored filament is behind a large share of extrusion complaints.

Problem: Diameter Inconsistency

• Symptom: extrusion looks uneven, with some regions better than others and no clear pattern.
• Solution: measure the filament with digital calipers at ten points along a metre. If variation exceeds the manufacturer's stated tolerance — typically ±0.02 to ±0.03 mm — replace the spool. No calibration compensates for filament that changes diameter as it feeds.

Problem: Wet Filament

• Symptom: distinct crackling or popping from the nozzle during printing, a rough surface and poor layer adhesion.
• Solution: most filaments are hygroscopic. Use a filament dryer and follow our guide to drying filament, then store sealed with desiccant.

PETG Under-Extrusion: Why It Behaves Differently

PETG generates more under-extrusion complaints than any other common material, and mostly for reasons that have nothing to do with the printer.

• It is more moisture-sensitive than PLA. A spool left open for a week absorbs enough water to pop and stutter. If PETG under-extrudes, dry it before you change a single setting — this fixes it more often than everything else combined.
• It needs a wider temperature window than people give it. PETG is more viscous than PLA at the same temperature. If you are running at the bottom of the range and printing fast, add 10 °C before you assume a clog.
• It responds badly to aggressive retraction. PETG is stringy, so people push retraction distance up to fight it — and then get a gap at the start of every line instead. Fix stringing with temperature and travel settings first, retraction last.
• It grips the nozzle exterior. PETG residue builds on the outside of the tip and drags on fresh extrusion. Brass-brush the nozzle hot between prints.
• Slow the first layer and use less squish than with PLA. Over-squishing PETG makes it stick to the nozzle rather than the plate, which reads as under-extrusion for the first few layers.

For structural PETG work, tougher PETG grades tolerate a wider processing window than basic ones, which makes them more forgiving while you dial the profile in.

Print with Confidence: A Final Summary

Solving under-extrusion is systematic, not mysterious. Check the cheap things first (slicer settings), then the physical components (nozzle, extruder, feed path), then the filament. Along the way, keep one principle in mind: change one variable at a time and print the same test object after each change. Two simultaneous adjustments tell you nothing about which one worked.

Think of your 3D printer as a precision tool that needs occasional calibration. Learning to diagnose under-extrusion is a core skill, and once you have it, consistent, strong prints stop feeling like luck. Our companion guide on fixing inconsistent extrusion covers the closely related case where flow varies rather than falls short.

FAQs About Under-Extrusion

Q1. Why does fast or hard retraction result in under-extrusion?

Retraction that is too fast or too long pulls molten plastic back into the cooler part of the hotend. When printing resumes there is a lag before plastic reaches the tip again, leaving small gaps and blobs after every travel move. On a direct-drive machine, retraction rarely needs to exceed about 1 mm. Tuning pressure advance addresses the same symptom at corners and line starts.

Q2. Could the wrong nozzle size in my slicer cause it?

Yes. If you fit a 0.6 mm nozzle but the slicer still thinks it is 0.4 mm, flow is calculated for the smaller passage and the printer deposits far less material than the wider bore lays down space for. The result is very weak, gappy parts.

Q3. What is heat creep and how does it cause under-extrusion?

Heat creep occurs when the hotend is not cooled adequately and heat travels up past the melt zone. Filament softens too early, swells, and binds in the heat break. Friction builds until flow degrades and eventually stops. It is the classic cause of a print that starts perfectly and starves an hour in.

Q4. Why does my print start well and lose material halfway through?

That points to a problem that develops over time. Look for a spool tangle tightening, heat creep building after an hour, or the extruder gear gradually filling with plastic dust and losing grip.

Q5. Why does the PTFE tube cause under-extrusion in Bowden printers?

Filament travelling back and forth wears a groove in the tube, especially near the fittings. The added friction stops the filament sliding freely, and the extruder motor no longer has the torque margin to push through it, producing consistent under-extrusion.

Q6. How do I fix PETG under-extrusion specifically?

Dry the spool first — PETG absorbs moisture quickly and wet PETG stutters. Then raise the temperature about 10 °C, reduce retraction distance rather than increasing it, and brass-brush residue off the outside of the nozzle. Only after those should you suspect a clog.

Q7. Is under-extrusion the same as missing layers?

No. Under-extrusion is continuous but insufficient flow. Missing layers are discrete gaps where a layer was skipped entirely, which can also be caused by motion system faults on the Z axis rather than anything to do with extrusion.